1. Field of the Invention
This application relates to systems and methods for clarifying liquids, especially water. More particularly, it concerns such systems and methods wherein a liquid stream is passed first upwardly through a non-fluidized, non-buoyant filter media and then sequentially passing the partially clarified liquid downward through a second filter media.
2. Description of the Prior Art
Although conventional sand filters are relatively inexpensive and effective in removing solid impurities from liquids, their capacity for solids retention is limited. Thus, the actual size of the pore spaces between the sand grains must be small to prevent passage of objectionable solids into the effluent and this limits the capacity of sand filters for storing solids. Accordingly, conventional clarifying systems using such filters have included sedimentation devices to protect the sand filters from rapid clogging by removing the bulk of the solids from the liquid being treated before being passed to the sand filter (see U.S. Pat. No. 4,051,039).
Instead of sedimentation devices, two stage filter systems have been developed. One class of the two stage systems uses upflow contact filtration in the both the first stage and the second stage (see U.S. Pat. No. 3,623,978). A second class of the two stage systems uses upflow contact filtration in the first stage and downflow contact filtration in the second stage. (see U.S. Pat. Nos. 4,547,286 and 4,793,934).
Difficulties have been encountered in the two stage filter systems that have limited their utility, namely, difficulty in stabilizing the upflow filter to achieve the required water quality and in preventing breakthrough of collected solids. It is known, by definition, that unstabilized media is fluidized and that fluidization will result in so-called "breakthrough" resulting in undesired loss or removal of solids to the effluent. Hence, expensive components such as media screens, grids and buoyant media have been employed in attempts to stabilize the upflow operation. The present invention provides improvements in such second class of two stage filter systems without need to utilize such expensive components to stabilize the media.